GEOGRAPHIC-DISTRIBUTION OF MITOCHONDRIAL-DNA VARIANTS AND THE HISTORICAL BIOGEOGRAPHY OF THE SPOTTED SALAMANDER, AMBYSTOMA-MACULATUM

I analyzed geographic partitioning of mitochondrial DNA (mtDNA) restri ction-site variants in the spotted salamander, Ambystoma maculatum. Tw o highly divergent and geographically separate genetic lineages were i dentified that differed by a minimum of 19 restriction sites (6% seque nce divergence). One of the lineages has a disjunct distribution with very closely related haplotypes occurring in Missouri, Arkansas, North Carolina, and Virginia. The other lineage is found in Michigan, Illin ois, and Alabama. The geographic separation of highly divergent mtDNA haplotypes, a pattern that was predicted based on the sedentary nature of these salamanders, is evidence for long-term barriers to gene flow . In contrast, the large-scale disjunction of very similar haplotypes suggests recent, long-distance gene how and does not match the phyloge ographic expectation for a small terrestrial vertebrate. I explain thi s potential contradiction in the level of importance assigned to gene flow by a scenario in which historical barriers to gene flow account f or the two divergent mtDNA assemblages, but stochastic sorting of ance stral polymorphism is responsible for the large-scale geographic disju nction. Ten of 16 populations collected in the Ozark Highlands were fi xed for the same haplotype. I attribute this lack of detectable variat ion to recent colonization of this area, a hypothesis that is supporte d by paleoecological data and demonstrates the potential benefits of c ombining data from paleobotany, geology, and other disciplines to reco nstruct the historical biogeography of a species.